djconnel wrote:
Yes -- I was plotting road racing bikes. The focus was on people who want a long/low position.
Numbers suggest that Cervelo geometry may not be as purely "racing" as Supersix or Scott Addict. Perhaps because they don't have two separate lines (Supersix vs Synapse) and they want to satisfy everyone at once.
Quote:
Stack-reach design philsophy is interesting. On the short side there's geometrical constraints. On the tall sides some companies seem to take a "granny gear" approach... one big one to catch as much of possible of the full statistical tail of tall riders. It's like when cassettes go 19-21-23-26-32 or similar. The spacing blows out at the big end.
BTW, I'm not sure why we're talking about stack-reach. Logically, it should be stack-ETT. Picture the rider as 3 straight links (legs - AB, torso - BC, arms - CD) with joints B, C, D between them. We want to fix top two links in space at a comfortable torso angle vs horizontal and at 90 degrees between torso and arms. That means targeting certain saddle-to-bars distance and saddle-to-bars drop (vector BD). We can't do that directly because rider inseam is a variable. Instead we start with stack and reach (vector AD) and fit BD = AD - AB. Which would be fine if AB were always pointed at a fixed angle.
The problem is that the direction of AB (seat tube angle) varies enough from bike to bike to be significant, and saddles have limited range of motion. If two bikes have identical stack and reach, but one has smaller seat tube angle, it will result in a greater saddle-to-bars distance at the same saddle offset for the same rider.
Using ETT in place of reach cancels out most of this effect, because we absorb most of the horizontal component of AB into it.
Here's a practical example. Look at size 50 Cannondale Evo in your plot (second green marker from the bottom). It would seem to be pretty long and low, with less stack but more reach than size 51 Cervelo (519,376 vs 530,369). Aggressive racing fit, right? Well, not exactly. Turns out that Evo has 74.5 deg seat tube, vs Cervelo's 73 deg. So, if you correct for seat tube angle by switching to stack/ETT, Evo is 519,520, Cervelo is 530,531 (size 51) / 505,516 (size 48). It's in line with Cervelo and actually slightly taller.